The most commonly used main reducer of rolling mill is the first or second grade cylindrical gear reducer. Speed ratio 7-8 is generally used as the dividing line of the first or second reduction gear. The center distance of the main gear box of the rolling mill is 1000-2400mm for the first-stage reducer and 2000-4200mm for the second-stage reducer. In the two-stage reducer, in order to make the load of the first-stage or second-stage uniform, the distribution principle of the center distance of the two stages is mainly to make the contact stress of the transmission tooth surfaces of the two stages approximately equal, and due consideration should be given to the reduction function having a small external size and weight. According to the above distribution principle, the ratio of the center distance between level 2 and level 1 is about 1.3-1.5. The smaller the ratio is, the smaller the secondary center distance is, and the smaller the profile size and weight of the reducer can be reduced if the contact stress of the tooth surface is allowed. When choosing the center distance of reducer of rolling mill, we should refer to the professional standard about the center distance.
As a gear reducer manufacturer, the gear of steel gear reducer is generally made of herringbone gear. As with the gear wheel, when the gear precision is above 8-8-7-dc, the hob retract slot should be left. In some two-stage reducers, in order to make the low-speed shaft gear and bearing load evenly, the first-stage transmission gear is usually composed of a pair of helical gears.
When the diameter of the gear is greater than 1,000mm, a notch is cast on the hub of the integral cast steel gear to reduce casting stress (figure 7-56). When the wheel is installed on the site, both sides of the wheel hub are heated with the fastening ring, and the cut is filled with babbitt alloy.
In order to make large gears have sufficient strength and save more valuable steel, the reducer of rolling mills usually adopts combined gears (figure 7-57). The wheel core of the combined gear is cast iron or steel, and the gear ring is forged steel or alloy. The height of the gear ring must be three times higher than that of the full tooth. When the gear is wide, double gear rings are available. Gear ring hot charging in heart, and hot charging temperature is 200-300 ℃, and the corresponding interference quantity is about one over one thousand the diameter of the ring gear to cooperate. In order to prevent possible loosening of the gear ring, in addition to sufficient overfilling during hot installation, there should be screw fixation on the joint surface of the gear ring and the wheel center. Furthermore, the spiral direction of the tooth should be such that the tooth ring can be attached to the convex shoulder of the wheel heart. In order to reduce the casting stress, the hub of the compound gear wheel core is often made with a cut. The disadvantage of combined gear is that there are many manufacturing processes and large amount of machining.